This study deals with the local spectroscopy and modification of semiconducting InGaP/GaAs/InGaAs quantum-well heterostructures by near-field scanning optical microscopy. The spatial distribution of the photoluminescence intensity in these structures is investigated and spatial nonuniformity of the photoluminescence is observed as a result of the nonuniform properties of the InGaP layers. It is shown for the first time that local quenching of the photoluminescence may be achieved by optically induced impurity diffusion near the quantum well, and this may be utilized to develop low-dimension semiconducting devices.